1. Field of the Invention
The present invention relates to a resonator for use in electronic apparatuses such as a communication apparatus, particularly to a resonator filter which passes only a desired band.
2. Description of the Related Art
A communication apparatus, which performs information communication by radio or wire, is constituted of various types of high-frequency component such as an amplifier, mixer, and filter. Such components include many high-frequency members having resonance characteristics. For example, a band-pass filter is constituted of a plurality of arranged resonance elements, and has a function of passing only a signal in a specific frequency band.
The band-pass filter for use in a communication system is required to have a skirt characteristic such that interference is not caused between adjacent frequency bands. Here, the skirt characteristic indicates the degree of attenuation extending to a blocking band from a pass band end. If a band-pass filter having a steep skirt characteristic is used in a radio apparatus, a communication frequency is effectively used in a communication system.
A method of realizing a filter having a steep skirt characteristic is reported, for example, in IEEE Transactions on Microwave Theory and Techniques, Vol. No. 48 (2000), pages 2519. This document discloses a method of using a large number of resonance elements constituting the filter as in a 32-pole filter. However, since a large number of resonance elements are used, this method has the drawback of enlarging a radio apparatus. Moreover, even in the filter in which a superconductor is used as a conductor constituting the resonance element to minimize conductor loss, the conductor loss becomes remarkable with the use of a large number of resonance elements. There is a problem that insertion loss increases.
As a method of realizing a filter having the steep skirt characteristic without using a large number of resonance elements, for example, there is a method of using a pseudo-elliptic function type filter, as reported in IEEE Transactions on Microwave Theory and Techniques, Vol. No. 48 (2000), pages 1240. When an attenuation pole is disposed in the vicinity of the pass band, this filter can realize a steep skirt characteristic with a small number of resonators. At this time, the filter including the attenuation pole is realized by using coupling between adjacent resonance elements and coupling between nonadjacent resonance elements which has a reverse phase. To realize the coupling of the reverse phase, one needs to be electric coupling and the other needs to be magnetic coupling. In a distributed element circuit, the electric coupling is obtained by coupling resonance element ends which are maximum voltage portions to each other. On the other hand, the magnetic coupling is obtained by coupling resonance element middle portions, which are maximum current portions to one another. That is, to realize the pseudo-elliptic function type filter in the distributed element circuit, both the electric coupling and the magnetic coupling need to be realized.
As a simplest example of a distributed element type half-wave length resonance element in which both electric coupling and magnetic coupling can be realized, there is a hair pin type resonance element reported in IEEE Transactions on Microwave Theory and Techniques, Vol. No. 46 (1998), page 118. An example of miniaturization in which the tip end of a hair pin is folded back to impart a capacitive property is reported in IEEE Microwave Theory and Techniques Symposium Digest, (1989), page 667. Moreover, an example of miniaturization in which the tip end of the hair pin is set to have a low impedance and the capacitive property is imparted is reported in Jpn. Pat. Appln. KOKAI Publication No. 5-299914 and IEEE Microwave Theory and Techniques Symposium Digest, (1997), page 713. An open loop type resonance element which is miniaturized by folding the tip end of the hair pin to impart the capacitive property is described in Electronics Letters, Vol. No. 31 (1995) page 2020. A meander open loop type resonance element which is constituted of a meander line and further miniaturized is disclosed in Electronics Letters, Vol. No. 32 (1996) page 563.
On the other hand, a substrate material forming the filter has a dispersion of thickness in the substrate plane or a dispersion of permittivity by crystal defects. Therefore, there has been a demand for a circuit which has a small dispersion of the filter characteristic by the dispersions of material characteristics. However, in reality, the filter characteristic sometimes deviates from a desired value by the dispersion of the material characteristic. In this case, there is a method of finely adjusting the length of each resonance element constituting the filter.
As described above, for the related-art small resonance element whose element tip ends are disposed opposite to each other, when the element tip ends are removed in order to finely adjust the length, the charged capacity is rapidly changed. This is because the element tip ends are disposed opposite to each other. Moreover, the resonance frequency rapidly changes. Therefore, there is a problem that fine adjustment is difficult.